Field of the Invention
This invention belongs to the field of subsea intervention tools for removing gas hydrate formations on the surface of equipment and connections used in oil exploration and production, more specifically, a tool for removing gas hydrate formations through high power laser beam radiation applied to the surfaces of the subsea equipment.
Description of Related Art
Oil and oil products play a leading role in industry. Oil is typically drawn from wells and shipped by pipeline. Depending on the location of the well and the destination of the oil, pipelines may be either onshore or offshore.
The flow of oil through a pipeline may result in the generation of different substances that block the flows of fluids. Thus, there may be a build-up of paraffin waxes, gas hydrates or other accretions of assorted materials, such as gravels or sand in the pipeline.
Furthermore, due to the low temperatures that prevail in the deep waters where oil is extracted, items of equipment used for oil drilling and production may be subject to external encrustations of matter such as gas hydrates resulting from leaks of light fractions of hydrocarbons which, under high pressures and at low temperatures (typically 4° C.), result in the formation of gas hydrates.
Gas hydrate or clathrate (which means ‘cage’ in Latin) is a crystal compound in which water molecules clump together with others, bonded with hydrogen, encapsulating molecules of gas such as methane and carbon dioxide. This type of intermolecular bonding is similar to the structure of ordinary ice, but the geometric arrangement produces cavities around gases that were originally held in solution in the water. Despite the absence of chemical bonds between the host (water) and guest (methane etc.), the stable structure of this clathrate means that when this substance is in a solid state, it does not melt at even temperatures well above 0° C., providing that the prevailing pressure and gas concentration are sufficiently high. These conditions are found in marine sediments along continental shelves at depths of more than around 500 meters. The guest molecule most commonly found in offshore gas hydrates is methane (CH4); the heavier hydrocarbons and molecules such as CO2 and H2S normally occur in proportions of less than 2%. While CO2, H2S and C2+ increase the fragmentation temperature of clathrate, the presence of salts dissolved in the water reduces its stability.
Gas leaks may occur in subsea oil exploration and production equipment (including light fractions of oil and other gases), seeping from these items of equipment. When gas leaks come into contact with moisture in the low temperatures and high pressures of a marine environment, this results in the formation of gas hydrates.
Laser sources generate laser light that is transferred to an optical head, comprised of optical elements such as lenses, windows, collimators etc. This laser light may be steered to this head by an optic fiber or any other wave guide able to steer the laser light. The optic fiber may be built into a coil or an umbilical.
The field presents some documents describing the use of laser tools for removing blockages from the inside of subsea pipelines carrying oil, with such blockages being caused by matter such as paraffins, gas hydrates and other substances inside or outside such pipeline.
Thus, U.S. Pat. No. 6,437,285 addresses a method and system for treating material covering the inner surface 52 using a pulsed laser ray 38. A laser source generates laser rays that are transferred to a laser head 100. A set of mirrors 380 in the head 100 distributes laser ray pulses in a circle 610 around the axis 54 of the head 100 and directs them towards the matter on the inner surface. Although this US Patent document addresses the removal of matter from the surfaces of offshore equipment by laser, the focus of the treatment is only the inner surface of the pipelines.
Republished U.S. Pat. No. RE33,777 addresses a process for removing matter with low heat conductivity, such as paint, grease, ceramics and similar, from a substrate through ablation without damaging the substrate. This process involves subjecting the matter to be removed to laser ray pulses or their equivalent at wavelengths whereby the matter to be removed is opaque to this wavelength, at a flow rate sufficient to break down the matter to be removed without causing any type of damage to the substrate or its surface. Claim 1 in this document defines the laser ray pulses as having duration greater than some three microseconds and less than some four thousand microseconds, with a repetition rate greater than one and less than around one thousand pulses per second.
In contrast, this application addresses the use of non-pulsed laser light, meaning continuous wave (CW), removing the matter through heating rather than by ablation, as set forth in U.S. Pat. No. RE33,777. The intensity of the laser light is controlled in order to maintain the equipment surface temperature at appropriate levels, avoiding any damage thereto.
This document explains that the selected wavelengths are those absorbed by the hydrate. When absorbing the radiation at a wavelength to which the hydrate is opaque, the hydrate formation or another substance is heated, resulting in its fragmentation, although when the hydrate or other substance breaks away from the outside towards the inside, there is a significant exchange of heat with the aqueous medium, thus imposing heavier energy demands.
In contrast, this application considers the water absorption spectrum, as well as the type of hydrate in question, in order to select the wavelength at which the fragmentation will occur with greater efficiency and less energy consumption. By selecting the wavelength to which the water and the hydrate are transparent or semi-transparent, the laser radiation spreads until meeting up with an opaque obstacle, which in this case will be the equipment or pipe in which the gas leak is occurring that gives rise to the hydrate. When reaching the equipment or pipe, the radiation will result in the heating in thereof, which in turn heats up the hydrate through conduction, breaking down the hydrate formation from the inside out.
Moreover, the U.S. Pat. No. RE33,777 document does not mention any anti-reflection coating on the system window that is in contact with the marine medium, matching the refraction index to that of the seawater as a way of obtaining maximum energy transmission, nor does it mention any optimization of the laser ray wavelength used in order to obtain maximum efficiency for removing matter such as gas hydrate or paraffin from the outer surface of subsea equipment.
The application of published US Patent. 20090205675A1 addresses an upgraded method and system for removing blockages from pipelines carrying hydrocarbons. As set forth in the technology addressed by this document, a laser head is placed in a pipeline carrying hydrocarbons that requires cleaning. The laser head is fitted with a laser ray. The laser head directs the laser ray to the portion of the pipeline to be cleaned. The laser source is linked optically through the laser head. The laser source is positioned in a transporter vehicle.
Although this document refers specifically to the use of laser radiation for the removal of unwanted matter such as paraffin or gas hydrates from the outer surfaces of pipelines carrying hydrocarbons, there is no mention of any anti-reflection coating on the tool window that is in contact with the marine medium, nor any other optimization of the laser wavelength used in order to obtain maximum efficiency for the removal of matter such as gas hydrate or paraffin from the outer surface of the subsea equipment.
Consequently, the documents constituting the state of the art generally address removal of blockages from the inside the pipes, and/or use lasers whose radiation is not optimized, as is the case with this tools and its associated method.